The metabolism of sucrose and lactose has been studied because of these sugars involvement in supporting the formation of dental caries in conjunction with the growth of Streptococcus mutants and Lactobacillus casei. Sucrose can be transported into S. mutants cells by the action of a PEP-PTS activity with the production of intracellular sucrose-6-phosphate (S6P). S6P has been chemically synthesized, and an enzyme catalyzing it is apparently identical with the previously described intracellular invertase. A fructokinase that catalyzes the ATP-dependent phosphorylation of fructose has been characterized as well. In L. casei it has been shown that lactose is transported into the cell by a specific lactose PEP-PTs; the resulting lactose-6-phosphate is cleaved by phospho-beta-galactosidase to glucose and galactose-6-phosphate. It has been shown that these first two steps of the metabolism of lactose are plasmid-coded. Several distinct lactose plasmids have been identified and their genetic potential and inter-relatedness is under investigation.